In communication systems information comprising speech or other data is transmitted via a high frequency signal. For these communication systems communication signals are modulated from the baseband onto a carrier signal via phase modulation, amplitude modulation or other modulation methods within a transmitter.
In general, at first a baseband is generated comprising the information being transmitted. For increasing the transfer rate it is advantageous to use the amplitude component as well as the phase component as information carrier. After modulating the signal onto a carrier signal the signal may be amplified to a adequate transmitting power.
It is very important to reduce power consumption of the used transmitters. In particular, the used power amplifiers, like class A or B amplifiers, dissipate much energy. Thus one method to realize reduction of power consumption is to make use of power amplifiers having a high efficiency, like switching amplifiers. For instance, class D or class E amplifiers can be used.
However, for employing switching amplifiers, suitable modulation methods are necessary since the switching amplifiers make high demands on linearity of the signal being amplified. Due to these high demands it is not possible to directly amplify a modulated signal which does not comprise a constant envelope. Thus appropriate modulation methods are needed, for instance, pulse width modulation (PWM) or pulse density modulation (PDM).
For controlling a switching amplifier, several methods are known. According to one method, the input of the amplifier is driven with a two-level PWM signal, which is generated by comparing the AM- and PM-modulated carrier signal with a triangular or saw tooth signal, which fundamental frequency is at least twice the carrier frequency. This poses the problem of a high switching frequency of the power amplifier and in particular the high linearity requirements regarding the triangular signal. Moreover, the carrier signal already should have been linearly AM-modulated before PWM conversion takes place.
Another possibility for driving a switching amplifier is to use a two-level PWM signal as the amplifier input signal, the PWM signal being generated by a band pass sigma-delta converter. The main disadvantage coming along with this solution is the even higher PA switching frequency, i.e. a significant over-sampling ratio.
According to another class of solutions for controlling a switching power amplifier the amplitude component is added via modulation of the supply voltage of the switching power amplifier. This modulation can be done via an efficient DC-DC converter. However, it is difficult to achieve a high modulation bandwidth and to suppress unwanted noise or ripple generated by the DC-DC converter at the same time.
It is one object of the present application to provide reduced power consumption of the apparatus. It is yet another object of the invention to improve the overall linearity of the apparatus.